Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the center line of the desired track during a read or write operation is referred to as "track following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially. In the absence of bias forces, the acceleration of the coil is proportional to the applied current. This current is applied by a power amplifier in response to a control input. If the input control is small enough, then the applied current is proportional to the control input and the power amplifier is nonsaturated; if the control input is too large, the applied current reaches a maximum possible value, and the power amplifier is saturated.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to efficiently move the head between tracks and to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline and to detect the track number or position sample.
In a disk file digital servo control system, a microprocessor utilizes a control signal algorithm to calculate a digital control signal based upon the digital values of certain state variables such as head position, VCM current and head velocity. The digital control signal is converted to an analog signal and amplified to provide input current to the VCM. Such a digital servo control system is described in assignee's U.S. Pat. No. 4,679,103 incorporated by reference herein, and is a system which, as part of the computation of the control signal to the actuator, makes use of the state estimator algorithm to estimate the state (i.e., position, velocity and acceleration or current input to the VCM) of the head. In this type of system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES, position sample, and the VCM input current, and computes, through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal to provide a power amplifier control signal. A power amplifier then generates a new VCM input current. The method of estimating the state of the physical plant to be controlled in such a digital servo control system requires the use of estimator constants, the derivation of which is described in Digital Control of Dynamic Systems, Franklin and Powell, Addison-Wesley Publishing Co. (1983), chapter 6, pages 131-139. In the case of a disk file, these estimator constants are dependent upon the values of certain physical parameters of the disk file, such as the moving inertia of the coil and head/arm assembly, the VCM force, or torque, factor (the torque applied to the coil per unit of input current), the gain of the VCM power amplifier, the PES gain and the time between PES samples (the PES sampling time).
The VCM current is fed back directly to the input to the power amplifier where it is summed with the analog control signal. This effectively changes the integrating power amplifier into a low pass filtering power amplifier, with linear input output characteristics. Thus, when the power amplifier is not saturated, the VCM current and state of the head can be easily modeled, based upon the linear characteristics of the servo system. During long seeks, when the power amplifier is saturated, the linear model is not accurate. During this phase of operation of the disk file, the servo system operates in open loop and the state estimator algorithm is modeled based upon the known VCM characteristics during saturation. The saturation model of VCM current incorporates the effects of both the back electromotive force (back EMF) and the coil current rise time.
The microprocessor in the digital servo control system determines which VCM current model to use by a determination of when the power amplifier is in saturation. In the example described in assignee's U.S. Pat. No. 4,914,644, the microprocessor determines power amplifier saturation based upon a value of the velocity error, which is the difference between the commanded velocity and the estimated velocity. When the velocity error is larger than a predetermined threshold, then the power amplifier is thought to be saturated and the saturation model is used.
The prior art patents discuss changes to the linear nonsaturated model resulting in modifications to the control signal so as to make the VCM more precise or to more quickly position the head correctly. In assignees' U.S. Pat. No. 4,835,633, which is incorporated herein by reference, the actual current input to the VCM is fed back to the nominal prediction model and, in the '644 patent, the actual current is estimated using a nominal current model and fed back to the nominal prediction model to estimate the next position of the head. The torque factor, as shown by the '633 patent, varies over the length of the stroke by up to 17% and can vary from device to device and over time in the same device, accounting for some modelling error. In addition, the assignee has found that the torque factor may vary by an additional 4% with temperature.
The '633 patent takes temperature change into account with respect to the torque factor as part of a measurement of the current state of the linear acceleration factor, but does not consider the change in the saturated seek mode, as the mode operates only at maximum current which is not subject to changes in the control.
Japanese published patent application 5-266615 measures the temperature directly in the magnetic circuit and measures the driver current flow directly, deducing the corresponding temperature. From a stored lookup table, the decrease in the torque constant is found and is used to adjust the gain of the linear servo loop, so that overall stability is improved.
The prior art patents, however, use only a nominal saturated model because the system is operating in open loop with the control signal at the level required to maintain the maximum current to the VCM actuator. The major problem described, for example, in assignee's U.S. Pat. No. 4,697,127, at col. 9, 11. 51-56 is that the coil resistance (and therefore, the maximum servo current) in saturated seek mode, indeed, even during nonsaturated seek, varies up to 30% with temperature. Therefore, the nominal model is built with sufficient safety to account for the worst case of this parameter.
Assignee's U.S. Pat. No. 5,119,250 attempts to overcome the difficulty of using the worst case model by modifying the point at which the current is switched from acceleration current to deceleration current, but still within a velocity profile which assumes the worst case conditions.
Whether or not the worst case model is used, an important problem in using the nominal model to predict the position and velocity of the VCM, is that the error between the predicted position and actual position at each servo sample time may become large. Typically, this error is integrated during the seek to provide a corrective "dynamic windage" term that is added to the control. If the error is large, this term will increase in size ("windage windup"), and the resulting control may be too large to "unwind" in time, resulting in overshoot of the target. Additionally, the windage estimate is typically used to provide the initial linear model static windage estimate such that the error will degrade the settle characteristics and likely the settle time of the seek.